Ionic conductivity and the formation of cubic CaH2 in the LiBH4-Ca(BH4)2 composite

Dadi Þorsteinn Sveinbjörnsson, Didier Blanchard, Jón Steinar Garðarsson Mýrdal, Reza Younesi, Rasmus Viskinde, Marit Dalseth Riktor, Poul Norby, Tejs Vegge

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Abstract

LiBH4–Ca(BH4)2 composites were prepared by ball milling. Their crystal structures and phase composition were investigated using synchrotron X-ray diffraction and Rietveld refinement, and their ionic conductivity was measured using impedance spectroscopy. The materials were found to form a physical mixture. The composites were composed of α-Ca(BH4)2, γ-Ca(BH4)2 and orthorhombic LiBH4, and the relative phase quantities of the Ca(BH4)2 polymorphs varied significantly with LiBH4 content. The formation of small amounts of orthorhombic CaH2 and cubic CaH2 in a CaF2-like structure was observed upon heat treatment. Concurrent formation of elemental boron may also occur. The ionic conductivity of the composites was measured using impedance spectroscopy, and was found to be lower than that of ball milled LiBH4. Electronic band structure calculations indicate that cubic CaH2 with hydrogen defects is electronically conducting. Its formation along with the possible precipitation of boron therefore has an effect on the measured conductivity of the LiBH4–Ca(BH4)2 composites and may increase the risk of an internal short-circuit in the cells.
Original languageEnglish
JournalJournal of Solid State Chemistry
Volume211
Pages (from-to)81-89
ISSN0022-4596
DOIs
Publication statusPublished - 2014

Keywords

  • Lithium borohydride
  • Calcium borohydride
  • Calcium hydride
  • Solid electrolyte
  • Ionic conductivity
  • X-ray diffraction

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